1. Field of the Invention
The present invention relates to a thin film magnetic head useful for PCM (pulse code modulation) recording and reproducing devices. More particularly, it relates to the construction of the lower magnetic film which forms a portion of the magnetic head structure.
2. Description of the Prior Art
A thin film magnetic head is generally considered to be superior in terms of mass production capability and also in its uniformity of magnetic properties since the coil conductors, the upper magnetic film and the insulating film that make up the magnetic circuit are formed by commercially available techniques such as sputtering. In addition, the miniaturization in size, particularly the reduction in the track and gap widths is facilitated in thin film magnetic heads since the patterning may be achieved by conventional photolithographic techniques.
The thin film magnetic head is also improved magnetically because the recording magnetic field in the head which takes part in the recording can be made quite intense to permit high density as well as high resolution recording. At the same time, the recording and reproducing device is reduced in size.
In general, in this type of thin film magnetic head of the prior art as shown in FIG. 5, there is a lower magnetic film 52 composed of a ferromagnetic material such as "Sendust" (an Fe-Al-Si alloy) located in superposition on a nonmagnetic substrate 51 formed, for example, of a ceramic material. With this type of structure, good magnetic recording and reproduction can be achieved even on a magnetic recording medium having a high coercive force while achieving high density recording and reproduction. Several turns of a lower coil conductor 54 are applied in a flat spiral over the lower magnetic film 52 and are separated therefrom by means of a first insulating film 53. Several turns of an upper coil conductor 56 are wound in a similar manner on the lower coil conductor 54 with the interposition of a second insulating film 55. In the embodiment shown, four turns of the lower coil conductor 54 and three turns of the upper coil conductor 56 are applied in the manner described above. In addition, the lower and upper coil conductors 54 and 56 are electrically connected to each other through a contact window 57. On top of the upper coil conductor 56, an upper magnetic film 59 formed, for example, of "Sendust" is provided with an intermediate third insulating film 58 to provide a predetermined track width. Hence, a closed magnetic loop is formed by the lower magnetic film 52 and the upper magnetic film 59 to permit magnetic recording and reproduction.
In the thin film magnetic head described above, there is a step-like difference in level necessarily formed in the upper magnetic film 59 due to the film thicknesses of the coil conductors 54 and 56. The thickness of the upper magnetic film 59a tends to be reduced on those stepped portions as compared to that on the flat portion. For this reason, magnetic saturation is likely to be achieved in the upper magnetic film during recording. Therefore, there is a problem arising in that it is difficult to produce a large recording magnetic field.
In order to prevent magnetic saturation of the upper magnetic film from occurring, it has been suggested to eliminate the step-like level difference to provide a substantially flat upper magnetic film 68 as shown in FIG. 6. Thus, there is provided a groove 69 for winding the coil conductors 63 and 65 therein, the groove being located on a lower magnetic film 66 on a magnetic substrate formed, for example, of a magnetic material such as a ferrite. In this groove 69 there are wound a lower coil conductor 63 and an upper coil conductor 65 utilizing intermediate insulating layers 62 and 64. The upper magnetic layer 68 is used in conjunction with an intermediate insulating layer 67 to provide for a predetermined track width.
The coil conductors 63 and 65 responsible for the step-like level difference in the upper magnetic film are wound within the groove 69 with the result that the upper magnetic film 68 becomes approximately flat to provide a construction with a uniform film thickness.
However, in the thin film magnetic head shown in FIG. 6, the lower magnetic film 66 tends to undergo magnetic saturation during signal recording with a resulting limitation on the recording magnetic field strength due to the splitting of the lower magnetic film 66 despite the fact that the substrate 61 is formed of a ferrite or similar magnetic material.
In the conventional thin film magnetic head, the lower or upper magnetic film is likely to undergo magnetic saturation during signal recording so that the recording field strength is drastically limited and good recording and/or reproducing properties are not obtained.